is there somebody able to tell me what is the difference between unofficial gs0a and official gs0c?

I used my 451@851 with gs0a and everything was fine. Then I loaded the new gs0c and the scan results of KProbe were worse.Using gs0a I had PImax 300 an POmax 20 at worst. Using gs0c the values rose up to PImax 600-800 and POmax 40-50. I use still the same media Verbatim +R @8x or Imation +R @8x. Both are originally labeled as 4x, but allow burning @ 8x.

I got pretty good scans with GS0C (I have a 451@851). Had a recent 8x burn (RICOHJPNR01, made in Taiwan) that scanned with an average of < 10 PI and < 1 PO.

Seems like, from what I've read, there are people who have had terrible results with the GS0C, and there are those who have been more fortunate and got great scans with the GS0C. My recommendation is that you try out GS0C and see for yourself.